The Neuropathology Core was developed to provide on-going support to the major investigators in the Drug Discovery project that is working to find a treatment for toxoplasmosis in the AIDS patient. The critical element served by this Core is to provide a means of measuring successful drug therapy by histologic methods so that there is not total reliance on survival data for evaluation of efficacy of a particular drug regime. The histologic changes in the brain and heart of immune-competent mice infected with toxoplasma are well known and characterized. Extensive experience with toxoplasma infection in immune-compromised mice and humans with AIDS or organs transplant has permitted us to delineate a continuum of histologic changes that correlate well with the clinical course of infection in a particular patient or mouse. The assessment of degree of involvement by infection in a particular patient or mouse. The assessment of degree of involvement by infection in a particular organ is always done in a blinded fashion so that identity of treated groups as well as the particular drug regime under investigation are not known to the person reading histological slides. In our past studies, true differences in various drug therapy on efficacy of containing the toxoplasma infection are easily discerned by evaluating the histology, particularly that of the brain. Immunoperoxidase staining for toxoplasma antigen will be of increasing importance as these studies continue because in the immunocomprised patient eradication of infection rather than simple control is essential, and this staining allows identification of dormant organisms in infected tissues.